The present invention pertains to a process and to a device for calibrating robot measuring stations, measuring robots and carried optical measurers calibrating, especially 3D sensors, within a measuring station for said workpieces, especially for vehicle body shells.
Robot measuring stations for workpieces, especially for vehicle body shells, in which one or more multiaxial measuring robots are equipped with optical measuring means, e.g., with 3D sensors, have been known from practice. There is a considerable calibration problem in the prior-art measuring stations and their measuring means. Only the measuring robot with its axes is usually calibrated by the using a suitable calibrating tool, with which the optical measuring means is replaced for this purpose, approaching suitable testing bodies, e.g., balls with a known position. The calibration of the monoaxial or multiaxial optical measuring means is also difficult. It has a defined working point, which is called Tool Center Point (abbreviated as TCP) or as Tool Coordinate System (abbreviated as TCS) in the robot program. Such monoaxial or multiaxial optical measuring means may be so-called 3D sensors, measuring cameras or the like. The working point is invisible in such optical measuring tools and it cannot be readily measured. Moreover, all optical sensors differ from one another in that all have a different sensor coordinate system. This is due to the system, on the one hand, and also due to the manufacture, on the other hand. The sensor coordinate system may vary even in identical sensor types. As a result, the measuring sensors are not readily interchangeable. If replacement is necessary in case of defect of a sensor, all working points must be tested in the robot program and possibly corrected one by one in a calibration operation. This diminishes the technical availability of the measuring station. The safety of the process is no longer guaranteed, either.
The measuring robot is calibrated once in the above-described manner prior to the measuring operation, and its axis errors are determined and compensated in the machine data and the control. In addition, a single-time alignment takes place with respect to the workpiece, e.g., by measuring with a higher-level measuring system. It is assumed in the measurement that the single-time adjustment operation is sufficient and the measuring robot will then have sufficient measuring accuracy in its entire work space. However, the measuring precision that can be achieved and the absolute accuracy are limited in practice and subject to the effects of errors, which occur during the operation over a longer period of time and can be attributed, e.g., to heat-dependent changes in the geometry of the robot or even to wear. The measuring precision that can be achieved cannot be guaranteed by a single-time adjustment or calibration.
The calibration of a processing robot with processing tools, e.g., vacuum grippers, welding torches or the like has been known from U.S. Pat. No. 5,297,238. This calibration operation is carried out by means of a camera system used temporarily, which is carried by the robot. The camera system is first calibrated here by the optical measurement of a preset surface with a plurality of defined points, the camera system being flanged to the hand of the yet uncalibrated robot and is brought into an approximately mutually perpendicular position in relation to this surface. Because of the inaccuracy of the robot, the measurement is performed several times and statistically evaluated. The reference of the robot to the environment of the cell is subsequently calibrated with the same camera system. In the third step, the position of the robot is calibrated in the working environment point by point by approaching and measuring a plurality of known points with the camera system and correcting any position deviations of the robot with point-related Cartesian offset. The tools used for the later work of the robot are subsequently also calibrated by means of the camera system.
Other processes for calibrating processing robots with processing tools have been known from the reference sources Loose D. C. et al.: xe2x80x9cPPA-A Precise, Data Driven Component Tool,xe2x80x9d IEEE Robotics and Automation Magazine, US, IEEE Service Center, Piscataway, N.J., Vol. 1, No. 1, Mar. 1, 1994, pages 6 to 12, and TSAI R. Y. et al.: xe2x80x9cA New Technique for Fully Autonomous and Efficient 3D Robotics Hand/Eye Calibration,xe2x80x9d IEEE Transactions on Robotics and Automation, U.S., IEEE Inc., New York, Vol. 5, No. 3, Jun. 1, 1989, pages 345 to 358. External camera systems and a camera carried next to the tool on the robot hand used to guide the tool are used for the calibration.
The object of the present invention is therefore to show an improved process with a device for calibration.
According to the invention, a process is provided for calibrating multiaxial measuring robots and carried optical measuring devices, especially 3D sensors for a measuring station for workpieces, especially for vehicle body shells. The calibration takes place in a measuring cascade with at least three calibration steps, wherein the optical measuring device with its working point, the measuring robot with its axes and then the assignment of the said measuring robot to the workpiece are calibrated one after another.
According to another aspect of the invention, a device is provided for the multistep calibration of multiaxial measuring robots and a carried optical measuring device such as a 3D sensor. The device is provided particularly for a measuring station for workpieces such as vehicle body shells. The device has a testing device for calibrating the working point of the optical measuring device, a calibrating body for calibrating the robot axes, and a calibrating device for the calibration of the assignment of the measuring robot to the workpiece.
The calibration in the measuring cascade with at least three calibration steps has the advantage that, on the one hand, the accuracy of calibration and measurement is substantially increased. An accuracy of approx. 0.05 mm can be reached in the individual calibration steps, which leads to an overall accuracy of approx. 0.1 mm for the entire measuring system comprising the measuring robot and the optical measuring device. Possible errors can be reliably recognized and specifically assigned to defined causes of error in the multiaxial calibration. This permits specific error corrective actions and makes possible the above-mentioned increase in accuracy.
A specific limiting of the causes of error is also possible with the measuring cascade. It is meaningful for this purpose for the first calibration step to concern the optical measuring device with its working point. Other causes of error, which originate from the measuring robot or from the measuring station, are eliminated in this calibration. The measuring robot with its axes is calibrated in the second step. The axis calibration of the robots is performed with the above-mentioned calibrated optical measuring tool. Effects of errors from the optical measuring tool are now ruled out. The measuring station has no effect, either. The third calibration step concerns the checking of the geometry of the station or cell and the assignment of the measuring robot to the workpiece or to the workpiece mount. The first two calibration steps make it possible to perform this checking of the cell geometry with the measuring robot and its optical measuring device. An additional and complicated external measurement can be eliminated. The two previous calibration steps offer high accuracy for the third calibration step as well.
The circumstance that the three-step calibration with the measuring cascade can be performed not only once at the beginning at the time of the setting up of the measuring station, but also at any time during the measuring operation, is of particular advantage. The calibration is simple, takes little time and requires only a low design effort. Due to the fact that calibration is possible at any time, the process safety of a robot measuring station is guaranteed to a sufficient extent for the first time ever. In particular, this also ensures the feasibility of the process to a sufficient extent. This is important especially because of the increasing accuracy requirements on the quality of workpieces and especially vehicle bodies.
Due to the high accuracy of measurement and calibration as well as the low design effort and time requirement, workpiece measurements can be performed more frequently. As a result, a measurement can be performed not only on the finished vehicle body but also some steps before during the manufacture of the components, so that errors and rejects can be recognized and eliminated in time. In particular, causes of errors can also be better assigned as a result in the workpieces or components and can be corrected more easily and more specifically. In addition, workpiece or component measurements can be carried out more frequently on a larger number of workpieces or components due to the lower time requirement.
There are further advantages in the possibility of having the calibration performed by mechanics or workmen without special measuring technical knowledge and without complicated numeric optimization programs. Furthermore, it is possible in robot measuring stations to implement a consistent CAD/CAM process chain for performing the measurement with measuring robots in conjunction with a data record. Without an exactly defined TCP, only a comparative measurement is possible on a sample workpiece, especially a sample body. In addition, the present invention makes possible the step from pure process monitoring in production (i.e., the checking whether a component is O.K. or not) to the robot-supported measurement of workpieces or components of any degree of complexity.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.